In recent years, an exhaust emission control device, which treats exhaust gas discharged from an internal combustion engine with a catalyst or a filter to reduce the discharge of toxic components, has gained importance as an environmental protection measure. For example, an exhaust emission control device has been known in which a diesel particulate filter (hereinafter referred to as “DPF”) is disposed in an exhaust pipe to collect particulate matters (hereinafter referred to as “PM”) discharged from the diesel engine. The DPF is regenerated by combusting and eliminating the accumulated PM regularly and, hence, can be used continuously.
The regeneration of the DPF is performed by increasing the temperature of the DPF to a certain temperature, for example 600° C. or more, at which the particulate matters can be combusted, when the quantity of the accumulated PM calculated based on a pressure difference across the DPF reaches a predetermined value. At this time, temperature increasing means includes means of post-injection, retardation of fuel injection timing, restriction of intake air and the like. However, such temperature increasing means cause deterioration of fuel consumption.
On the other hand, as a regeneration temperature is increased, the combustion rate of the PM is increased and, hence, the regeneration is finished in a short time so that the deterioration of fuel consumption caused by the regeneration of the DPF can be reduced. However, if the temperature of the DPF is too high, there is a possibility of causing the DPF to be broken or an oxidation catalyst supported by the DPF to deteriorate. Hence, to prevent the deterioration of fuel consumption and to regenerate the DPF safely, the temperature of the DPF needs to be maintained near a predetermined temperature. Therefore, usually, the temperature of exhaust gas upstream or downstream of the DPF is sensed and the temperature increasing means is operated such that the detected temperature becomes a target temperature.
For example, Japanese patent document JP-11-101122A discloses the following control method: the temperature of exhaust gas upstream of the DPF is detected as a DPF temperature and when the DPF temperature becomes higher than a predetermined temperature (for example, 500° C.), a temperature increasing operation is interrupted and when the DPF temperature becomes lower than the predetermined temperature (for example, 500° C.), the temperature increasing operation is performed. However, changes in the temperature of the DPF caused by the temperature increasing operation develop time lag. Hence, the method disclosed in JP-11-101122A, in which the temperature increasing operation is performed or interrupted after temperature changes are sensed, causes large variations in temperature and hence is hard to keep the temperature of the DPF near the target temperature.
The inventors of the present invention studied a method for controlling an operation of increasing a DPF temperature with higher accuracy and proposed a method of controlling the quantity of operation of increasing temperature by controlling the ratio between a time period during which a temperature increasing operation is performed and a time period during which the operation is interrupted (hereinafter referred to as “time ratio between the performance and interruption of a temperature increasing operation”). An example of such is disclosed in Japanese Patent Application No. 2003-94851. The time ratio is expressed by the ratio of a period τ1 during which the temperature increasing operation is performed to a predetermined basic period τa, as shown, for example, in FIG. 12A. By performing the post-injection of the temperature increasing means at this time ratio, the quantity of supply of HC can be controlled stepwise or continuously to control the DPF temperature optimally, as shown in FIG. 12B. Further, by controlling the state of injection suitably (for example, by correcting injection timing and the quantity of injection suitably) such that the torque when the temperature increasing operation is performed is equal to the torque when the temperature increasing operation is interrupted, torque shock can be prevented at the time of switching the temperature increasing operation.
However, there has been presented a problem that when the state of combustion varies from the initial shipment due to age deterioration of an injection system, a torque difference is caused by switching between performing the temperature increasing operation and stopping the operation to develop a periodic torque shock at intervals of the basis of the time ratio. In this case, there is a possibility that the periodic torque shock may be transmitted to a driver to cause the deterioration in drivability.